I can buy either a carbon x1 4th Generation with Windows 7 or 5th generation with Windows 10 for the same price. I generally would go with the newer machine however I really like Window 7 and have heard iffy things about the windows 10 interface. Is there much of a difference between 4th and 5th generation? What would you do?

We are using X1C4 (Generation-4, circa 2016) on Win7 Pro, bought it before the X1C5 (Gen-5, circa 2017) came out.
We plan to use Win7 Pro on our X1C4 until its support ends in the year 2020.

If you plan to use only Windows and not Linux, then your choice essentially boils down to this:
~X1C5 for its "future proof" USB-C ports (not so sure about USB-C charging using other branded USB-C adapters). Slightly faster faster CPU which supports 4k video (you need to double check on this part). Same max 1 TB SSD storage, same max 16 GB RAM.

~X1C4 for its Windows 7 support (last model to support this). Supports Linux. No USB-C ports.

X1C5 - from what we have read, differences to X1C4:
~Top tier 1 TB SSD PCIe-NVMe has OPAL 2.0 support (supports full disk encryption). [X1C4 has 1 TB but no OPAL 2.0 support]
~Intel HD Graphics 620 supports 4k video, we think. [X1C4 does not support 4k video--as if you really need this]
~New: USB-C (USB 3.1) ports and USB-C charging. X1C5 using USB-C charging is slightly slower than X1C4, but nothing to worry about.
~Does X1C5 comes with Windows 10 Pro Signature Edition?? If this is true, you may have trouble installing Linux onto this machine. [X1C4 supports Linux]

X1C4 - differences to X1C5:
~Windows 7 support!
~Supports Linux.Note: If you plan to use Windows 7 on your X1C4, we strongly advise you to buy the X1C4 with Win7 downgrade rights directly from Lenovo. We bought ours from a dealer with close links to Lenovo, and they ended up making a custom made Product Recovery disk set just for us!!
Reason? Apparently the X1C4 made in the 2nd half of 2016 had fingerprint readers which were incompatible with Win7 (no Win7 drivers at the time). Therefore in the country where we bought our X1C4, X1C4 made in 1st half of 2016 supports Win7. X1C4 made in the 2nd half of 2016 does not support Win7 !

X1C5 IMO. W10 is not as bad as most people make it out to be. I still prefer linux, but W10 offers better productivity and security over W7. For example, the addition of virtual desktops can really save a lot of time and keep you organized. Also the slight increase in hardware speed is always welcome.

mirc, your X1C5 you mentioned is Win10 Home, which does not support the Hyper-V (Virtual Machine).
(Hyper-V requires Win10 Pro).

On the other hand, X1C4 you mentioned had Win7 Pro which has Virtual-PC (Virtual Machine).
I think you can still upgrade your X1C4's Win7 Pro to Win10 Pro, which would assure you of Win10 Pro's Hyper-V (Virtual Machine) capability.